PROJECT SUMMARY Salmonella enterica (S. enterica) serovars are pathogenic bacteria that cause significant morbidity and mortality in humans worldwide. Nontyphoidal Salmonella (NTS) gastroenteritis is the most prevalent of the clinical syndromes associated with S. enterica serovars. The global burden of NTS gastroenteritis is estimated to account for 93.8 million cases of disease annually, with 155,000 deaths. S. enterica serovars associated with NTS gastroenteritis such as S. enterica serovar Typhimurium (S. Typhimurium) invade intestinal epithelial cells and induce a secretory response in the intestinal epithelium that initiates the recruitment of innate immune cells, including neutrophils and mononuclear phagocytes. While this inflammatory response helps limit S. Typhimurium invasion in the long term, recent studies have shown that S. Typhimurium exploits gut inflammation to edge out competing microbes in the intestinal lumen and establish infection. Specifically, the inflammatory response changes the composition of the intestinal microbiota and suppresses its growth. Furthermore, the inflammatory response provides nitrate and tetrathionate, by-products of the inflammatory response that are used by S. Typhimurium as electron acceptors for anaerobic respiration and growth in the inflamed intestine. The generation of inflammation-derived tetrathionate is dependent on the production of reactive oxygen species (ROS). Neutrophils are major producers of ROS and have been shown to play an important role in S. Typhimurium-induced colitis. The generation of inflammation-derived nitrate, which is the preferred electron acceptor used by S. Typhimurium for anaerobic respiration and growth in the inflamed intestine, is dependent on Nos2, which encodes inducible nitric oxide synthase (iNOS). This enzyme catalyzes the production of nitric oxide (NO) during inflammation. The cellular source(s) of iNOS and, therefore, NO- derived nitrate used by S. Typhimurium for anaerobic respiration and growth in the inflamed intestine, have yet to be identified. We have shown recently that inflammatory monocytes recruited into tissues following S. Typhimurium infection are major producers of NO and that their ability to produce NO is dependent on iNOS. Based on these published findings and the preliminary results described in this application, we hypothesize that inflammatory monocytes play a key role in S. Typhimurium-induced colitis. The research proposed in this application will test this central hypothesis and open up a new area of research on the role of inflammatory monocytes in immunity and host defense. Completion of the proposed research will define the role of inflammatory monocytes in S. Typhimurium-induced colitis and provide new avenues for targeted modulation of inflammatory monocytes as a new approach to the development of novel therapeutics to overcome infectious and digestive diseases such as NTS gastroenteritis, Crohn's disease and ulcerative colitis.